fix pathops fuzz failures

src/pathops/SkPathOpsConic.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkIntersections.h"
 #include "SkLineParameters.h"
 #include "SkPathOpsConic.h"
 #include "SkPathOpsCubic.h"
@@ skipping 30 matching lines @@
         return 1;
     }
     return 0;
 }
 
 SkDVector SkDConic::dxdyAtT(double t) const {
     SkDVector result = {
         conic_eval_tan(&fPts[0].fX, fWeight, t),
         conic_eval_tan(&fPts[0].fY, fWeight, t)
     };
+    if (result.fX == 0 && result.fY == 0) {
+        if (zero_or_one(t)) {
+            result = fPts[2] - fPts[0];
+        } else {
+            // incomplete
+            SkDebugf("!k");
+        }
+    }
     return result;
 }
 
 static double conic_eval_numerator(const double src[], SkScalar w, double t) {
     SkASSERT(src);
     SkASSERT(t >= 0 && t <= 1);
     double src2w = src[2] * w;
     double C = src[0];
     double A = src[4] - 2 * src2w + C;
     double B = 2 * (src2w - C);
@@ skipping 71 matching lines @@
     SkDConic dst = {{{{ax / az, ay / az}, {bx / bz, by / bz}, {cx / cz, cy / cz}}}, w };
     return dst;
 }
 
 SkDPoint SkDConic::subDivide(const SkDPoint& a, const SkDPoint& c, double t1, double t2,
         SkScalar* weight) const {
     SkDConic chopped = this->subDivide(t1, t2);
     *weight = chopped.fWeight;
     return chopped[1];
 }